How many Queen's speeches can be voted down in a row?

There's a new attack aimed at the SNP now. It's in the form of the question: "would you vote down a Labour Queen's Speech?" with the inference that such a speech was written purely by the Labour party and without consulting the SNP.

It is then followed up by the assertion that if the SNP did vote down a Labour Queen's Speech, this would herald in a Tory government, and then the questioners leave it there. In the viewers mind, of course, this leaves the impression that voting down a Labour Queen's Speech would automatically result in a Tory government that could never, ever be voted out for the next five years.

But, in such a scenario, what is stopping the SNP and Labour (and other like-minded parties) from then voting down a Tory Queen's Speech? Given the House of Commons works entirely by votes, and each party can vote as freely as they wish, I see no reason why the SNP can't vote down a Labour Queen's Speech and then, a few weeks later, vote down a Tory Queen's Speech.

What would happen after both the Labour and Tory Queen's Speeches have been voted down?

Presumably, the Queen would invite the SNP to form the government (assuming they're the third-largest party). Then we can hear complaints that Ed Miliband would be holding Nicola Sturgeon to ransom and that there would be a risk of Labour writing SNP's budget.

So I think this new line of attack is nonsense. Any number of Queen's Speeches can be voted down, thus forcing like-minded parties to work together to create a common Queen's Speech that will gather the support of a majority of the House of Commons.

If Labour do refuse to work with the SNP, we might yet see Alex Salmond become Prime Minister.

Note: Nicola Sturgeon is not standing to be elected as an MP at Westminster, so she couldn't become Prime Minister.